[Meta] Enable multi-hop migration of data through snapshot-restore

[kartg]

This meta issue tracks feature work that enables customers to migrate their data from a cluster running on a legacy version to the latest version of OpenSearch, through the snapshot-restore mechanism.

• Enable searchable snapshot support for older versions (ES 6.0) - [Proposal] Enable searchable snapshots to read legacy versions OpenSearch#5451
• Expand the above functionality to support even older versions (say ES 5.0)
• Investigate re-indexing of old snapshots on the updated cluster

This method could also be used for upgrading clusters in-place, but would result in downtime

[setiah]

Enabling a legacy snapshot to restore on latest version of OpenSearch would enable users to migrate directly from previous versions of Elasticsearch and OpenSearch to the latest tip via the familiar Snapshot Restore process. I like the idea of solving 1 & 3 tasks first w.r.t. ES 6.x+ versions and later moving to expand this support with subtask 2 to more legacy versions. @kartg

[setiah]

While snapshot restore is great option for lift-and-shift migrations, it would be exciting to see this functionality expanded to in-place upgrades as well as it provides zero downtime option for version migrations. Could we also track that as part of Multi-version meta issue?

[kartg]

Related - opensearch-project/OpenSearch#5443

[kartg]

Ownership of the extended compatibility of searchable snapshots has been moved to the core opensearch repo - opensearch-project/OpenSearch#5948

We will revisit reindexing of snapshots at a later time. Closing the issue.